Check data for null before trying to manipulate it. Closes #1391

Signed-off-by: Adam Warner <me@adamwarner.co.uk>

scripts/pi-hole/js/groups-clients.js

@@ -210,6 +210,11 @@ function initTable() {
     },
     stateLoadCallback: function () {
       var data = utils.stateLoadCallback("groups-clients-table");
+      // Return if not available
+      if (data === null) {
+        return null;
+      }
+
       // Reset visibility of ID column
       data.columns[0].visible = false;
       // Apply loaded state to table